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04.21.16         tutorial & training videos  

How to Calculate A Cameras Buffer Depth & Write Speed


So in my latest Epic Shootout with the A6300 vs the Canon 80D, I stumbled upon a simple way to figure out the how much memory a buffer holds, as well as how fast the camera can write to a memory card. If you are unfamiliar with buffer performance concepts, you might want to check out my video on how buffers work: How Buffer Systems Work

There are a few assumptions that would have to be true for this to work:

1. You are using a memory card that allows information to written faster than the camera's maximum write speed, otherwise the memory card will slow this down and you would actually be calculating the card write speed. In my case, I am using San Disk Extreme Pro U3 cards which are designed to have a maximum write speed of 95MB/s.

2. We have to assume there are two parts to this that are each needed to figure the other one out, the buffer filling stage and the buffer full stage.

3. We have to assume that as soon as we take the first picture, the camera begins writing to the memory card, in the case of the Canon 80D and A6300, there is a write indicator light that basically suggests this is the case.

4. You need a way to accurately measure how long a camera is in a “buffer filling stage” and a “buffer full and writing to the memory card stage”, I decided to use the audio in my time line to measure how long each of these were.

5. You need to know the file sizes written for each of these 2 stages.

With the Sony, this was fairly easy to figure out because it’s compressed RAW files are consistently ~24MB each (though occasionally there is some minor variation).

Here is how it worked for the Sony

1. On Full Burst, RAW, Sony shoots at 11 frames per second, for 2 full seconds, or 22 total files, each at 24MB each, or a total of 528 MB of files captured during those 2 seconds.

2. Once the buffer is full, the camera allows you to take another picture, as the buffer is clearing. So the assumption is you can take another picture as soon as that space becomes available, but if we know that we took 7 additional RAW files over 4.8 seconds, we can calculate:

7 x 24MB = 168MB written in 4.8 seconds, or 35MB/s. Write speeds are typically written in bits, which we find by multiplying that by 8, so the write speed of the A6300 is ~ 280 Mb/s.

Coming back to the total number of MB captured during those 2 seconds of burst, we subtract what should be the total amount a data written during those 2 seconds, or 35MB x 2 = 70MB.

528MB - 70MB = ~ 458 MB Buffer Space

I realize that there may be some software limitations that Sony put in to protect the camera for various reasons, but this gives us a close idea of what we are dealing with buffer depth and write speed wise.

I expect there will be some who will freak out saying that the either the Sony or the Canon or even the card are limited to 100mb/s or 50mb/s because that is what it says on the video codecs, etc, but remember, "sustained write speeds" for video are very different than maximum read/write speeds for image files and that is an important distinction.

Rumors are that Sony's next full frame will have an unlimited buffer, and this can be accomplished simply by having write speeds greater than the maximum frames per second x file size. Under those conditions, the buffer should never fill, assuming you have a super fast memory card of course.

Exercise:

So now that you know how to calculate buffer depth and write speed, here at the measurements for the Canon:

26 shots, for a total of 734 MB over a "buffer filling space" of 3.9 seconds.
Once the buffer was “full” it shot 13 images, totaling 363.95 MB, over 4.53 seconds.

What did you get?

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